FLOODPLAIN SEDIMENTATION AND SENSITIVITY - SUMMER 1993 FLOOD, UPPER MISSISSIPPI-RIVER VALLEY

Patterns of overbank sedimentation in the vicinity of, and far removed from, levee breaks that occurred in response to the >100 year, summer 1993 flood in the upper Mississippi River valley are elucidated. Two suites of overbank deposits were associated with the failure of artifi cial levees within a 70km long study reach. Circumjacent sand deposits are a component of the levee break complex that develops in the immed iate vicinity of a break site. As epitomized by the levee break comple x at Sny Island, these features consist of an erosional, scoured and/o r stripped zone, together with a horseshoe-shaped, depositional zone. At locales farther removed from the break site, the impact of flooding was exclusively depositional and was attributed to the settling of su spended sediment from the water column. The overall picture was one of modest scour at break sites and minimal suspended sediment deposition (<4 mm) at locales farther removed from the breach. Downriver from th e confluence with the Missouri River, suspended sediment deposition wa s of a similar magnitude to that observed within the study reach and l evee break complexes exhibited a similar morphology, but scour at brea k sites was greatly enhanced and the excavated sand formed extensive d eposits on the floodplain surface. The different erosional response wa s probably engendered by the higher sand content and reduced aggregate cohesion of the floodplain soils downriver from the confluence with t he Missouri River. A qualitative comparison serves to highlight the in fluence that the resistance threshold may have on the sensitivity of f loodplains bordering large low-gradient rivers to high magnitude flood s.